The present invention relates to a digital video disk (DVD) system. More particularly, it relates to a sub-picture restoring method and apparatus in a DVD system which restores characters or graphics defined in the DVD standard and displays the restored result. This application for a DVD sub-picture restoring method and apparatus is based on Korean Patent Application No. 96-26258 which is incorporated by reference herein for all purposes.
A DVD has been developed, which can provide higher picture quality than that of a compact disk (CD). The DVD is convenient to use and handle, as is the CD, and can be used as a reproduction-only medium for movie to computer media. Also, various proposals have been made in conjunction with the DVD standard, one of which is use of sub-picture data. The sub-picture data is graphic data which can be displayed in the form of a caption on a screen, in which a basic unit is defined in the form of FIG. 1.
FIG. 1 shows the structure of sub-picture data defined in the DVD standard. The sub-picture data includes a header having general information with respect to a sub-picture, pixel data which is produced by compressing the actual sub-picture, and a display control sequence table (DCSQT). The DCSQT includes display control sequences (DCSQ0, DCSQ1, . . . ) of a frame unit having display control commands in conjunction with sub-pictures and time information in conjunction with execution of these commands, in the form of a table.
Here, the pixel data includes 2-bit data for discriminating the four types of patterns. Therefore, each pixel becomes a background pixel around a character when the 2-bit data is "00," a character pattern pixel when the former is "01," a first emphasis pixel of the edge of the character when the former is "10," and a second emphasis pixel when the former is "11." The above pixel data is encoded by compression based upon the number of sequential repetitions of the same pattern type of the pixels via a run-length coding operation.
The DCSQT contains information about the size and position of horizontal and vertical display areas on a screen necessary for decoding sub-picture data of the compression-encoded bit stream. In more detail, the DCSQ of the frame unit constituting the DCSQT contains information about display time of the sub-picture, compulsory display having no relation with the display time, sub-picture display end time, color designation with respect to each pixel, and a pixel contrast value for determining a mixture ratio with respect to the particular other image data. The DCSQ also contains the display area information of the sub-picture data, the position information with respect to the compressed pixel data, and the information which can alter color and contrast with respect to the pixel belonging to a particular range.
The information which can alter color and contrast with respect to the pixel belonging to a particular range will be described below with reference to FIG. 2. FIG. 2 shows the pattern of a command CHG.sub.-- COLCON for altering the color and contrast of the sub-picture pixel. The contents of the command are largely divided between a line control information LCINF with respect to a particular range of a line unit and a pixel control information PCINF with respect to a particular range of a pixel unit. A maximum of eight kinds of pixel control information PCINF can exist with respect to a single line control information LCINF, while various kinds of line control information LCINF can exist with respect to a single command CHG.sub.-- COLCON. The LCINF contains the start line number, the end line number and the number of the pixel control information PCINF existing in the range with respect to the range in which the color and contrast are identically altered. The PCINF contains the start pixel number representing the horizontal start position existing in the range with respect to the range in which the color and contrast are identically altered, and the color and contrast values which are altered with respect to each pixel until the start pixel number of the next pixel control information is input.